At the time, only four states had a requirement of two doses at school entry, the agency noted, but by 2012 that had risen to 36. As well, all but one state required at least one dose of the vaccine before school entry.

To estimate the effect of the change, the agency compared two-dose coverage among 7-year-olds in 2006 and 2012, using data from six Immunization Information System (IIS) sentinel sites -- Michigan, Minnesota, North Dakota, New York City, Wisconsin, and part of Oregon.

In 2006, the agency report said, two-dose coverage in the six sites ranged from 3.6% to 8.9%, but rose to a range of 79.9% to 92% in 2012.

Uptake was rapid, the agency report said, reaching an average of 72.4% across the sentinel sites by 2009.

"These increases suggest substantial progress" since the 2007 recommendation, the agency report said, and wider use of the school entry requirements might help reach the Healthy People 2020 target of 95% of kindergarten students having received two doses of varicella vaccine.

The one-dose varicella vaccination program, which began in 1996, led to reduced varicella incidence, hospital admissions, and mortality ranging from 70% to 90%.

The routine two-dose program has been accompanied by reported declines in varicella incidence and outbreaks ranging from 67% to 76%, the agency report said.

The agency cautioned that the report used census-based denominators, which included children with a history of varicella disease, for whom vaccination would not be indicated. That might have led to an underestimate of the level of vaccine protection.

But as a final word of the caution the report noted that the six sentinel sites might not be representative of other cities and states.

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